U.S. patent application number 12/692312 was filed with the patent office on 2010-05-27 for fuel cells with hydrophobic diffusion medium.
This patent application is currently assigned to GM GLOBAL TECHNOLOGY OPERATIONS, INC.. Invention is credited to Chunxin JI, Vinod Kumar.
Application Number | 20100129534 12/692312 |
Document ID | / |
Family ID | 37896646 |
Filed Date | 2010-05-27 |
United States Patent
Application |
20100129534 |
Kind Code |
A1 |
JI; Chunxin ; et
al. |
May 27, 2010 |
FUEL CELLS WITH HYDROPHOBIC DIFFUSION MEDIUM
Abstract
Diffusion media for use in PEM fuel cells are provided with
silicone coatings. The media are made of a porous electroconductive
substrate, a first hydrophobic fluorocarbon polymer coating adhered
to the substrate, and a second coating comprising a hydrophobic
silicone polymer adhered to the substrate. The substrate is
preferably a carbon fiber paper, the hydrophobic fluorocarbon
polymer is PTFE or similar polymer, and the silicone is moisture
curable.
Inventors: |
JI; Chunxin; (Pennfield,
NY) ; Kumar; Vinod; (Pittsford, NY) |
Correspondence
Address: |
Harness Dickey & Pierce, P.L.C.
P.O. Box 828
Bloomfield Hills
MI
48303
US
|
Assignee: |
GM GLOBAL TECHNOLOGY OPERATIONS,
INC.
Detroit
MI
|
Family ID: |
37896646 |
Appl. No.: |
12/692312 |
Filed: |
January 22, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11250197 |
Oct 14, 2005 |
|
|
|
12692312 |
|
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Current U.S.
Class: |
427/115 |
Current CPC
Class: |
Y02P 70/50 20151101;
Y02E 60/50 20130101; H01M 8/0245 20130101; Y02T 90/40 20130101;
H01M 8/0234 20130101; H01M 8/0239 20130101; H01M 2250/20 20130101;
H01M 8/04291 20130101 |
Class at
Publication: |
427/115 |
International
Class: |
H01M 4/88 20060101
H01M004/88 |
Claims
1. A method for making a diffusion medium for use in a PEM fuel
cell, comprising: coating a porous conductive substrate with a
hydrophobic fluorocarbon polymer, and contacting the fluorocarbon
coated substrate with a silicone composition comprising a moisture
curable silicone resin.
2. A method according to claim 1, wherein coating comprises
contacting the substrate with a coating composition comprising a
solvent and particles of a hydrophobic fluorocarbon polymer,
removing the solvent, and sintering the particles.
3. A method according to claim 2, comprising loading the substrate
with 1 to 20% by weight of the fluorocarbon particles, based on the
total weight of the coated substrate after sintering.
4. A method according to claim 1, comprising loading the substrate
with 3 to 10% by weight of the fluorocarbon particles, based on the
total weight of the coated substrate after sintering.
5. A method according to claim 1, wherein contacting comprising
spraying the silicone composition onto the substrate.
6. A method according to claim 1, further comprising applying a
microporous layer to one side of the substrate, wherein the
microporous layer comprises fluorocarbon polymer and conductive
particles.
7. A method for improving the high humidity performance of a PEM
fuel cell stack, the stack comprising a plurality of PEM fuel
cells, the fuel cells comprising a cathode, an anode, a
polyelectrolyte membrane disposed between the cathode and the
anode, flow fields adjacent the electrodes, and a
fluoropolymer-coated diffusion medium disposed between at least one
of the cathode and the cathode flow field and the anode and the
anode flow field, the method comprising contacting the
fluoropolymer carbon diffusion medium with a silicone composition
comprising a moisture curable silicone resin.
8. A method according to claim 7, wherein the diffusion medium is
disposed between the cathode and the cathode flow field.
9. A method according to claim 7, comprising providing the
diffusion medium with a hydrophobic coating on areas of the
diffusion medium not covered by fluorocarbon polymer.
10. A method according to claim 7, comprising contacting the
diffusion medium with the silicone composition prior to assembly of
the stack.
11. A method according to claim 7, comprising identifying a
low-performing cell during operation of the stack, contacting the
diffusion medium of the low-performing cell with the silicone
composition, and continuing operation of the stack.
12. A method according to claim 11, comprising removing the
diffusion medium of the low-performing cell from the stack,
treating the diffusion medium by contacting it with the silicone
composition, and reassembling the stack with the treated diffusion
medium.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a divisional of U.S. patent application
Ser. No. 11/250,197 filed Oct. 14, 2005. The entire disclosure of
this application is incorporated herein by reference.
FIELD OF THE INVENTION
[0002] This invention relates to fuel cells with hydrophobic
diffusion medium. In particular, the invention relates to fuel cell
diffusion media having hydrophobic silicone coatings.
BACKGROUND OF THE INVENTION
[0003] Fuel cells are increasingly being used as a power source for
electric vehicles and other applications. An exemplary fuel cell
has a membrane electrode assembly (MEA) with catalytic electrodes
and a proton exchange membrane (PEM) formed between the electrodes.
Gas diffusion media play an important role in PEM fuel cells.
Generally disposed between catalytic electrodes and flow field
channels in the fuel cell, they provide reactant and product
permeability, electronic conductivity, and heat conductivity, as
well as mechanical strength needed for proper functioning of the
fuel cell.
[0004] During operation of the fuel cell, water is generated at the
cathode based on electrochemical reactions involving hydrogen and
oxygen occurring within the MEA. Efficient operation of a fuel cell
depends on the ability to provide effective water management in the
system. For example, the diffusion media prevent the electrodes
from flooding (i.e., filling with water and severely restricting
O.sub.2 access) by removing product water away from the catalyst
layer while maintaining reactant gas flow from the bipolar plate
through to the catalyst layer.
[0005] The gas diffusion media are generally constructed of carbon
fiber containing materials. Although carbon fibers are themselves
relatively hydrophobic, it is usually desirable to increase the
hydrophobicity or to at least treat the carbon fiber with a more
stable hydrophobic coating. Adding a hydrophobic agent such as
polytetrafluoroethylene (PTFE) to the carbon fiber diffusion media
is a common process for increasing the hydrophobicity. This process
is normally done by dipping carbon fiber papers into a solution
that contains PTFE particles and other wetting agents, such as
non-ionic surfactants.
[0006] Fuel cell stacks can contain a large number of fuel cells
depending on the power requirement of the application. For example,
typical fuel stacks have up to 200 individual fuel cells and more.
Because the fuel cells in the stacks operate in series, a weakness
or poor performance in one cell can translate into poor performance
of the entire stack. For this reason, it is desirable for every
fuel cell in the stack to operate at high efficiency.
[0007] Because typical fuel stacks contain so many individual fuel
cells, it has been observed that, even with a high degree of
reliability of manufacture of diffusion media, it is sometimes
observed that an individual or several diffusion media will have
less than optimum performance, especially at a high relative
humidity. When that occurs, a fuel stack containing such a fuel
cell will generally exhibit less than optimum performance. Thus,
diffusion media with enhanced hydrophobic properties and methods
for producing them that lead to consistent results among hundreds
of fuel cells in a single fuel stack would be an advance in the
art.
SUMMARY OF THE INVENTION
[0008] In one aspect of the invention, silicone coatings are
provided on diffusion media for use in fuel cells, such as PEM fuel
cells. The diffusion media are made of a porous conductive
substrate, a first hydrophobic fluorocarbon polymer coating adhered
to the substrate, and a second coating comprising a hydrophobic
silicone polymer adhered to the substrate. In various embodiments,
the porous conductive substrate is a carbon fiber paper or other
conductive substrate commonly used in a PEM fuel cell, and the
hydrophobic fluorocarbon polymer is a hydrophobic polymer such as
polytetrafluoroethylene (PTFE).
[0009] In various embodiments, the second coating is applied to a
conductive substrate on which the first coating has already been
applied; the first coating adheres to a substrate over a major part
of the surface area of the substrate, and the second coating (the
silicone polymer) adheres to an area or areas of the substrate that
are not completely covered by the first coating. The second coating
is preferably applied by contacting the substrate containing the
first coating with a silicone composition. Preferably, the silicone
composition contains components that cure to form the hydrophobic
silicone polymer adhering to the substrate. In a preferred
embodiment, the silicone polymer system is curable by the action of
moisture and typically at room temperature.
[0010] Performance of PEM fuel stacks containing up to 200 or more
individual fuel cells containing such diffusion media is improved
and found to be more reliable, by virtue of the improved
hydrophobic nature of the individual diffusion media in the stack.
Accordingly, methods are provided for making the diffusion media
and for improving the performance of fuel cell stacks containing
individual fuel cells containing the media. The methods involve
contacting a conductive porous substrate on which a hydrophobic
fluorocarbon polymer is adhering with a silicone composition
comprising, in a preferred embodiment, a moisture curable silicone
resin.
[0011] In various embodiments, fuel cell stacks are assembled
wherein each fuel individual fuel cell of the stack contains a
diffusion medium coated with a hydrophobic silicone as described.
In some embodiments, operation of a fuel cell stack is improved by
first identifying any individual fuel cell that is performing
poorly by virtue of having a diffusion medium that is too
hydrophilic, and treating that diffusion medium of that fuel cell
with the silicone composition as described.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a schematic diagram of a fuel cell stack
[0013] FIG. 2 shows current voltage curves of treated diffusion
media.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0014] In one embodiment, a diffusion medium suitable for use in a
PEM fuel cell is made of a porous conductive (i.e.,
electroconductive) substrate having a first coating comprising a
hydrophobic fluorocarbon polymer adhered to substrate. A second
coating comprising a hydrophobic silicone polymer is also adhered
to the substrate. In various embodiments, the substrate comprises a
carbon fiber based diffusion medium, such as a carbon fiber paper.
In typical embodiments, the hydrophobic fluorocarbon polymer is a
hydrophobic material such as polytetrafluoroethylene (PTFE).
[0015] In another embodiment, a method for making a fuel cell
diffusion medium for use in a PEM fuel cell comprises first coating
a porous conductive substrate with a hydrophobic fluorocarbon
polymer. After the substrate is coated with the hydrophobic
fluorocarbon polymer, the fluorocarbon polymer coated substrate is
then contacted with a silicone composition comprising a moisture
curable silicone resin. In various embodiments, the hydrophobic
fluorocarbon polymer is applied to a loading of 1 to 20% by weight
based on the total weight of the substrate, and the silicone is
applied from about 0.01 to about 5%, preferably from about 0.1 to
2% by weight, based on the total weight of the coated substrate.
The method can further be applied to diffusion media that contain a
microporous layer coating on one side of the conductive substrate.
The microporous layer contains a fluorocarbon polymer and
conductive particles, and generally has pore sizes much smaller
than the pore sizes on the side of the substrate not coated with
the microporous layer.
[0016] In another embodiment, fuel cells are provided that contain
hydrophobic diffusion media as described herein. In a further
embodiment, fuel cell stacks are provided that contains a plurality
of the fuel cells.
[0017] In another embodiment, a method is provided for improving
the high humidity performance of a PEM fuel cell stack. The stack
contains a plurality of PEM fuel cells, each of the fuel cells
containing a cathode, an anode, and a polyelectrolyte membrane
disposed between the cathode and the anode, and further containing
flow fields adjacent the electrodes (i.e., the anode and cathode).
A fluoropolymer-coated diffusion medium is disposed between at
least one of the electrodes and its flow field, that is, at least
one of cathode and the cathode flow field and the anode and the
anode flow field. That is to say, the individual fuel cells contain
a diffusion medium on the cathode side and/or the anode side. The
method involves contacting the fluoropolymer coated diffusion
medium with a silicone composition that contains a moisture curable
silicone resin. In various embodiments, the method results in the
application of a hydrophobic silicone coating on areas of the
fluorocarbon polymer coated diffusion medium that for one reason or
another, including random or unpredicted variations in fluorocarbon
coating processes, contain areas not completely coated with
fluorocarbon polymer, which areas are therefore more hydrophilic
than the rest of the diffusion medium.
[0018] In various embodiments, the method is carried out by
operating a fuel cell stack and identifying any individual cells in
the stack that are not performing as expected. In some embodiments,
the fuel cell performing poorly is removed from the stack and the
diffusion medium treated as described with a silicone coating.
Thereafter the fuel cell stack is reassembled.
[0019] Fuel cell stacks contain a plurality of fuel cells, the
number of individual cells depending on the power and voltage
requirements of the application. In automotive use, typical fuel
cell stacks contain 50 or more individual fuel cells and can
contain up to 400, 500, or even more. Power requirements in various
applications can also be met by providing a number of modules
comprising individual fuel cell stacks. The modules are designed to
work in a series to provide adequate power and are sized to fit
within the available packaging.
[0020] FIG. 1 is an expanded view showing some details of the
construction of a typical multi-cell stack, showing just two cells
for clarity. The bipolar fuel cell stack 2 has a pair of membrane
electrode assemblies (MEA) 4 and 6 separated from each other by an
electrically conductive fuel distribution element 8, hereinafter
bipolar plate 8. The MEA's 4 and 6 and bipolar plate 8 are stacked
together between stainless steel clamping plates or end plates 10
and 12 and end contact elements 14 and 16. The end contact elements
14 and 16, as well as both working faces of the bipolar plate 8,
contain a flow field of a plurality of grooves or channels 18, 20,
22, and 24 respectively, for distributing fuel and oxidant gases
(i.e. hydrogen and oxygen) to the MEA's 4 and 6. Non-conductive
gaskets 26, 28, 30, and 32 provide seals and electrical insulation
between several components of the fuel cell stack. Gas permeable
conductive materials used as gas diffusion media are typically
carbon/graphite diffusion papers 34, 36, 38, and 40 that press up
against the electrode faces of the MEA's 4 and 6. The end contact
elements 14 and 16 press up against the carbon graphite diffusion
media 34 and 40 respectively, while the bipolar plate 8 presses up
against the diffusion medium 36 on the anode face of MEA 4, and
against carbon graphite diffusion medium 38 on the cathode face of
MEA 6. Oxygen is supplied to the cathode side of the fuel cell
stack from storage tank 46 by appropriate supply plumbing 42, while
hydrogen is supplied to the anode side of the fuel cell from
storage tank 48, by appropriate supply plumbing 44. Alternatively,
ambient air may be supplied to the cathode side as an oxygen source
and hydrogen may be supplied to the anode from a methanol or
gasoline reformer. Exhaust plumbing (not shown) for both the
hydrogen and oxygen sides of the MEA's 4 and 6 will also be
provided. Additional plumbing 50, 52, and 54 is provided for
supplying liquid coolant to the bipolar plate 8 and end plates 14
and 16. Appropriate plumbing for exhausting coolant from the
coolant bipolar plate 8 and end plate 14 and 16 is also provided,
but not shown.
[0021] Individual fuel cells contain a proton exchange membrane
disposed between electrodes. The electrodes are an anode and a
cathode for use in carrying out the overall production of water
from fuel containing hydrogen and an oxidant gas containing oxygen.
In various embodiments, the electrodes contain carbon support
particles on which smaller catalyst particles (such as platinum)
are disposed, the carbon and catalyst supported generally on a
porous and electroconductive material such as carbon cloth or
carbon paper. Suitable electrodes are commercially available; in
some embodiments, the anode and cathode are made up of the same
material.
[0022] The electrically conductive porous material or substrate for
use as the diffusion media in the invention is in general a porous
planar flexible material that may be wetted by water or other
solvents associated with solutions of polymers as described below.
In various embodiments, the porous material (also called a sheet
material) is made of a woven or non-woven fabric or paper.
[0023] In a preferred embodiment, the sheet material is made of a
carbon fiber substrate such as carbon fiber paper. Carbon
fiber-based papers may be made by a process beginning with a
continuous filament fiber of a suitable organic polymer. After a
period of stabilization, the continuous filament is carbonized at a
temperature of about 1200.degree. C.-1350.degree. C. The continuous
filaments can be woven into carbon cloth or chopped to provide
shorter staple carbon fibers for making carbon fiber paper. These
chopped carbon fibers are made into carbon fiber paper sheets or
continuous rolls through various paper making processes.
Thereafter, in an illustrative embodiment, the carbon fiber papers
are impregnated with an organic resin and molded into sheets or
rolls. The woven carbon cloth and the molded carbon paper sheets or
rolls are then carbonized or graphitized at temperatures above
1700.degree. C. Suitable carbon fiber-based substrates are
described, for example in Chapter 46 of Volume 3 of Fuel Cell
Technology and Applications, John Wiley & Sons, (2003), the
disclosure of which is helpful for background and is incorporated
by reference. In various embodiments, the substrates take the form
of carbon fiber paper, wet laid filled paper, carbon cloth, and dry
laid filled paper.
[0024] Carbon fiber papers may be thought of as a non-woven fabric
made of carbon fibers. Carbon fiber paper is commercially available
in a variety of forms. In various embodiments, for example, the
density of the paper is from about 0.3 to 0.8 g/cm.sup.3 or from
about 0.4 to 0.6 g/cm.sup.3, and the thickness of the paper is from
about 100 .mu.m to about 1000 .mu.m, preferably from about 100
.mu.m to about 500 .mu.m. Typical porosities of commercially
available papers are from about 60% to about 80%. Suitable carbon
fiber papers for use in fuel cell applications as described herein
are available for example from Toray Industries USA. An example of
commercially available carbon fiber paper from Toray is TGPH-060,
which has a bulk density of 0.45 gm/cm.sup.3 and is approximately
180 microns (micrometers) thick.
[0025] In one aspect, the hydrophobic fluorocarbon polymer is one
that will settle out of an emulsion or precipitate out of a
solution under the evaporating conditions described below.
Preferably, the polymer deposited onto the sheet material is one
that will remain stably in contact with the portions of the sheet
during conditions of its use in the eventual end application, such
as a diffusion medium in a fuel cell. The compatibility or
stability of the polymer in contact with the substrate may be
enhanced by certain post-curing steps where the coated sheet
material is heated to a high temperature (e.g., 380.degree. C. for
PTFE) to fix the structure of the polymer on the sheet
material.
[0026] The fluorocarbon polymer generally imparts a hydrophobic
character to the substrate sheet material where the polymer is
deposited. By convention, the polymer is considered to render the
surface of the substrate hydrophobic if the surface free energy of
the polymer material is less than the surface free energy of the
sheet material itself. Surface free energy of the polymer and the
sheet material may be measured by and correlated to the contact
angle of water in contact with the polymer or sheet material,
respectively. For example, if the contact angle of water on the
polymer is greater than the contact angle of water on the sheet
material, then the polymer may be considered a hydrophobic
material. If the contact angle of water on the polymer is less than
the contact angle of water on the sheet material, the polymer may
be considered as a hydrophilic polymer.
[0027] Suitable fluorocarbon polymers include fluorine-containing
polymers, made by polymerizing or copolymerizing one or more
monomers that contain at least one fluorine atom. Non-limiting
examples of fluorine-containing monomers that may be polymerized to
yield suitable fluorocarbon polymers include tetrafluoroethylene,
hexafluoropropylene, vinylidene fluoride, perfluoromethyl vinyl
ether, perfluoropropyl vinyl ether, and the like. The presence of
fluorine-carbon bonds is believed to be responsible for the
hydrophobic nature of these polymers.
[0028] A preferred fluorocarbon polymer is polytetrafluoroethylene
(PTFE). PTFE is preferred in some embodiments because of its wide
availability and relatively low cost. Other fluorine-containing
polymers may also be used. Suitable fluorocarbon polymers include
without limitation PTFE; FEP (copolymers of hexafluoropropylene and
tetrafluoroethylene); PFA (copolymers of tetrafluoroethylene and
perfluoropropylvinylether); MFA (copolymers of tetrafluoroethylene
and perfluoromethylvinylether); PCTFE (homopolymers of
chlorotrifluoroethylene); PVDF (homopolymers of vinylidene
fluoride); PVF (polymers of vinylfluoride); ETFE (copolymers of
ethylene and tetrafluoroethylene); and THV (copolymers of
vinylidene fluoride, hexafluoropropylene, and tetrafluoroethylene).
Aqueous dispersions of these and other fluorocarbons are
commercially available, for example from DuPont. The dispersions
may be conveniently prepared by emulsion polymerization of
fluorine-containing monomers and other monomers to form the
polymers. Alternatively, the dispersions may be made by combining
polymer powder, solvent, and surfactants. The polymer dispersion
may comprise from 1-90% by weight of the fluorocarbon polymer with
the balance comprising water and surfactant. For example, DuPont
T30 PTFE product is available containing 60% by weight PTFE
particles.
[0029] In a non-limiting procedure, the fluorocarbon polymers are
applied to the porous electroconductive substrate by wetting the
substrate in a wetting composition including the polymer and a
liquid. The liquid is also referred to as a "solvent". In some
embodiments, the wetting composition is provided in the form of an
emulsion. Solutions may also be used. In some embodiments, the
wetting compositions contain surface-active materials or other
agents to hold the polymer in solution or suspension, and/or to aid
in wetting the substrate. For example, an emulsion used to wet the
sheet material may include from 1 to about 70% by weight particles
of a hydrophobic polymer such as polytetrafluoroethylene. In
various embodiments, ranges of 1% to 20% are preferred. In a
preferred embodiment, the polymer composition contains
approximately 2% to 15% of the polymer solids by weight.
[0030] The liquid is preferably aqueous (water or water mixture),
and may further comprise organic liquids. Generally, non-ionic
surfactants are used as wetting agents, with the result that no
metal ions will be left in the carbon fiber diffusion media after
the wetting agents are decomposed during high temperature
treatment. Non-limiting examples of surfactants include nonylphenol
ethoxylates, such as the Triton series of Rohm and Haas, and
perfluorosurfactants.
[0031] In a preferred embodiment, a substrate is prepared by
applying the fluorocarbon polymer composition to at least one
surface of the substrate. The polymer composition may be applied to
both sides of the substrate by immersing the porous substrate
(e.g., a carbon fiber paper or cloth) into a fluorocarbon
dispersion, by spraying both sides of the sheet-like substrate, or
by other suitable means. In a typical procedure, the substrate is
dipped into the fluorocarbon dispersion and removed after a time of
soak. In other embodiments, the polymer composition may be applied
to only one surface of the substrate, for example, by spraying,
vapor deposition, and the like. Exposure of the substrate to the
fluorocarbon polymer dispersion occurs for a time sufficient to
provide the substrate with the proper amount of fluoropolymer. A
wide range of loadings of PTFE or other fluorocarbon may be applied
to the carbon fiber substrate. In some embodiments, it is desirable
to incorporate about 2 to 30% fluorocarbon polymer by weight of the
diffusion medium, measured after the drying and other steps noted
below. In other embodiments, at least 5% by weight polymer is
incorporated into the diffusion medium. Typically the substrates
may be dipped or immersed in the fluorocarbon dispersion for a few
minutes to obtain an appropriate loading of fluorocarbon on the
substrate. In various embodiments, the dispersion contains from 1%
to 50% by weight of fluorocarbon particles. Dispersions having
concentrations of particles in the preferred range may be made by
diluting commercial sources of the dispersions as necessary to
achieve the desired concentrations. In a non-limiting example, a
dispersion containing 60 weight percent (%) PTFE may be diluted 20
times with de-ionized water to produce a dispersion containing 3%
by weight PTFE particles.
[0032] The time of exposing the substrate to the fluorocarbon
polymer dispersion is long enough for resin particles to imbibe
into the pores of the carbon fiber paper or cloth, yet short enough
to be an economically viable process. Generally, the time of
soaking and the concentration of the fluorocarbon polymer
particles, as well as the nature of the resin, may be varied and
optimized to achieve desired results.
[0033] After applying the fluorocarbon polymer composition to at
least one surface of the substrate, it is preferred to remove
excess solution before further processing. In one embodiment, the
substrate may be removed from the liquid dispersion and the excess
solution allowed to drip off. Other processes are possible, such as
rolling, shaking, and other physical operations to remove excess
solution.
[0034] The diffusion medium is preferably then dried by removing
the solvent. Removal of the solvent may be achieved by a variety of
methods, such as convective heat drying or infrared drying.
[0035] In addition to the hydrophobic fluorocarbon polymer coating
discussed above, the invention provides in various embodiments for
application of a further surface layer or layers. The most common
is referred to as a microporous layer, conductive particles mixed
with a polymeric binder. In various embodiments, the microporous
layer is applied as a paste to the substrate, and may be applied
before or after the hydrophobic fluorocarbon polymer coating, or it
may be applied to a surface or side of the medium not covered by
the fluorocarbon polymer. As noted, the paste contains conductive
particles and preferably particles of a polymeric binder.
Non-limiting examples of conductive particles include carbon
particles such as, without limitation, carbon black, graphite
particles, ground carbon fibers, and acetylene black. The polymeric
binder is preferably made of a fluorocarbon polymer or fluororesin
such as discussed above with respect to the first coating on the
substrate. In this regard a preferred fluorocarbon polymer for
making the paste is PTFE. In various embodiments, the paste is
applied to the substrate to form the microporous layer by
conventional techniques, such as doctor blading, screen printing,
spraying, and rod coating.
[0036] In practice, the paste is made from a major amount of
solvents and a relatively lesser amount of solids. The viscosity of
the paste can be varied by adjusting the level of solids. The
solids contain both the carbon particles and the fluorocarbon
polymer particles in a ratio by weight of from about 9:1 to about
1:9. Preferably, the weight ratio of carbon particles to
fluorocarbon polymer is from about 3:1 to about 1:3. The
fluorocarbon particles are conveniently supplied as a dispersion in
water. An exemplary paste composition contains 2.4 grams acetylene
black, 31.5 mL isopropanol, 37 mL deionized water, and 1.33 g of a
60% by weight dispersion of PTFE in water. This paste has a weight
ratio of acetylene black to fluorocarbon polymer, on a dry basis,
of about 3:1.
[0037] The paste is applied onto the dried porous substrate to
provide a microporous layer that extends from the surface into the
interior of the paper. In various embodiments, the microporous
layer is about 5 to about 20% of the thickness of the paper. For
example, with a typical paper 200 microns thick, the microporous
layer is from about 10 to about 30 microns thick above the surface
of the paper. Penetration of the microporous layer into the bulk of
the paper can range up to about 100 .mu.m, and depends on the
viscosity of the paste. The amount of paste to apply to a paper can
be determined from the density of the solids, the area of the
paper, and the thickness of microporous layer desired. In various
embodiments, a paste is applied to a paper at an areal loading of
about 1.0 to about 2.5 mg/cm.sup.2, based on the weight of the
solids in the paste.
[0038] The microporous layer preferably has a pore size of the
carbon agglomerates, i.e., between about 100 and about 500 nm, as
compared with 10 to 30 microns pore size for carbon fiber paper
substrates. The microporous layer provides effective removal of
liquid water from the cathode catalyst layer into the diffusion
media. For this reason, the diffusion medium is preferably
installed in the fuel cells with the microporous layer side toward
the cathode. The microporous layer also aides in reducing
electrical contact resistance with the adjacent catalyst layer. The
properties of the microporous layer can be varied by adjusting the
hydrophobicity of the polymeric binder and the particle and
agglomerate structure of the conductive particles.
[0039] After optional application of the microporous layer, the
substrate is preferably sintered by heating at a temperature high
enough to melt the particles of polymeric binder and coalesce the
microporous layer. In the case of PTFE containing microporous
layers, a temperature of 380.degree. C. has been found to
sufficient.
[0040] After the first coating that comprises a hydrophobic polymer
is applied and adhered to the substrate, a second coating
comprising a hydrophobic silicone polymer is adhered to the
substrate. Any first coating and/or microporous layer is to be
sintered prior to applying the second (silicone) coating.
[0041] The second coating results in a hydrophobic silicone layer
being applied to the substrate. A hydrophobic silicone layer is one
that gives a contact angle of water of greater than 90.degree..
Although the invention is not limited by theory, it is believed
that the second coating and the resulting hydrophobic silicone
coating sticks or adheres to portions of the substrate that are not
adequately covered by PTFE or other fluorocarbon polymers. Without
the silicone coating, these areas of the substrate would remain
more hydrophilic than the other coated areas and become more
hydrophilic over time during fuel cell operation. It is believed
that such hydrophilic areas, which result from incomplete coverage
of the substrate by the fluorocarbon polymer, lead to poor
performance, especially at high humidity, of cells that contain
such coated substrates as diffusion media. One reason for this may
be that the hydrophilic areas on the incompletely covered substrate
tend to retain water rather than repel water. In addition,
heterogeneous surface properties of the gas diffusion media facing
the flow channels makes it more difficult to remove water slugs in
the gas flow channels, which can result in uneven gas flow
distributions among different cells. As a result, water tends to
accumulate and inhibit the electrochemical reactions of certain
cells, which results in so-called low performing cells. By covering
any such small hydrophilic areas on the fluorocarbon coated
substrate with additional hydrophobic polymer, such as the second
hydrophobic silicone polymer coating, the diffusion medium is
rendered more homogeneously hydrophobic. As a result, the
performance of fuel cells containing the substrates, especially at
high relative humidity, is improved.
[0042] In various embodiments, the second coating is adhered to the
substrate by contacting the diffusion medium with a silicone
composition that contains a curable silicone resin, preferably one
that is curable by the action of moisture. In various embodiments,
the silicone composition is applied to the substrate by dipping,
spraying, or other means. Conveniently, the silicon composition
contains a solvent in addition to the silicone resin components. If
the silicone resin is moisture curable, it is preferred to use a
solvent other than water. Preferably the solvent is one that does
not interfere with the cure of the silicone or with the application
of the silicone coating onto the substrate. Methylene dichloride
has been found to be a suitable solvent.
[0043] In a preferred embodiment, the curable silicone resin is one
in which the cure is activated by contact with moisture. In various
embodiments, the curable silicone resin contains a silicone
prepolymer and a crosslinker. The prepolymer and the crosslinker
contain functional groups that react with one another preferably
activated by the presence of water or moisture, to form a
crosslinked or cured silicone polymer. The resulting polymer,
formed on the surface of the substrate, and preferably on areas of
the substrate not adequately coated by the fluorocarbon polymer, is
hydrophobic and acts to repel water and prevent accumulation on the
cathode of the fuel cell. As a result, performance of the fuel
cell, especially at high relative humidity, is enhanced.
[0044] Advantageously, after application of a moisture curable
silicone resin as described above, curing occurs upon exposure to
moisture in air, and can further continue after assembly into the
fuel cell and fuel cell stack during operation of the fuel cell
stack that produces water.
[0045] Illustrative silicone prepolymers include those that are
represented by the following structure
##STR00001##
[0046] In the structure R.sup.1 and R.sup.2 are the same or
different and independently represent aliphatic or aromatic groups.
In various embodiments, the aliphatic or aromatic groups R.sup.1
and R.sup.2 comprise alkyl, aryl, perfluoroalkyl, and
perfluoroaryl, preferably having from 1 to 20 carbons. In an
embodiment, R.sup.1 and R.sup.2 independently contain from 1 to 6
carbons. The aliphatic and aromatic groups R.sup.1 and R.sup.2 can
contain ether linkages, as long as the resulting polymer upon cure
is hydrophobic enough to enhance performance of a fuel cell
containing a coated diffusion medium. For example, the R.sup.1 and
R.sup.2 groups can independently comprise polyethers such as
polypropylene oxides and polyperfluoroolefin ethers. The groups
X.sup.1 and X.sup.2 are chemical moieties that provide the silicone
prepolymer with the ability to react with complementary functional
groups on the crosslinker to form the cured hydrophobic polymer on
the surface of the substrate. Preferred X.sup.1 and X.sup.2 groups
include hydrogen and lower alkyl having 1 to 6 carbons. The
silicone prepolymer has a molecular weight determined by the value
of n, which represents the average number of repeating siloxane
units in the prepolymer. Generally, n is greater than 2 and is
typically about 200-1500 for room temperature vulcanized materials
and 3000-11000 for heat cured materials. Typical values of n range
from about 10 to about 1000.
[0047] In various embodiments, the crosslinker has a structure
defined by the following
Si Y.sub.m R.sup.3.sub.4-m (2)
where the R.sup.3 are independently a group that does not
participate in the crosslinking. Non-limiting examples include
aliphatic and aromatic groups such as discussed above for the
prepolymer. In preferred embodiments, R.sup.3 is selected from
among C.sub.1-6 alkyl, and C.sub.6-10 aryl groups. Preferred
R.sup.3 groups include methyl and hydrogen. Y is a functional group
that is reactive with the X.sup.1O and X.sup.2O groups of the
prepolymer to form covalent bonds to crosslink and cure the resin.
Preferred functional groups for element to Y includes hydroxyl and
alkoxy, especially alkoxy of 1 to 6 carbon atoms. In a preferred
embodiment, the Y group is methoxy or ethoxy. In the crosslinker, m
is greater than 1 and is preferably 2 or greater. In a preferred
embodiment, m is greater than 2 and preferably less than 4.
[0048] Preferably, the groups X.sup.1 and X.sup.2 and Y are such
that crosslinking is enhanced by the action of water. For example,
when X.sup.1 and X.sup.2 are hydrogen or alkoxy and Y is hydroxyl
or alkoxy, reaction of water acts to enhance the crosslinking
reaction.
[0049] The silicone resins optionally further comprise suitable
additives such as fillers, other auxiliaries, chain transfer agents
for controlling polymerization, and the like. Suitable silicone
resins are commercially available. Non-limiting examples include
Dow Corning 3140 adhesive. The hydrophobic silicone coating is
applied by contacting the substrate with a silicone composition as
described above. A variety of methods can be used to carry out the
contact such as dipping, spraying, rolling, doctor blading, and the
like. In a non-limiting example, the substrate is dipped into a
dilute solution of the curable silicone resin. The dilution of the
resin and the method of application are chosen so as to apply
sufficient silicone polymer to make the resulting diffusion medium
hydrophobic and improve cell performance, especially at high
relative humidity. It has been found acceptable to add about 1% of
silicone polymer, based on the total weight of the diffusion
medium.
[0050] In various embodiments, the methods described above can be
used to carry out a kind of repair on a fuel cell stack where
certain cells in the stack are exhibiting low performance at high
relative humidity operation conditions and/or low current operation
conditions (e.g. much lower gas flow rate and thus hard to remove
water slugs in the gas flow channels). As discussed, such fuel cell
stacks normally contain a large number of individual fuel cells
depending on the power required. Typically fuel cell stacks contain
20 to 500 cells, 50 to 500 cells, 100 to 400 cells, or 200 to 400
cells. Because the cells are installed in series, any low
performing individual cell will affect the performance of the fuel
stack. In one embodiment, the methods above can be used to repair
or remediate an individual fuel cell operating in such a fuel
stack. In this method, the low performing cell in the stack is
identified and the diffusion medium of the cell is treated by
contacting it with a curable silicone resin. Upon reassembly of the
fuel cell and the fuel cell stack, it is observed that the high
humidity performance of the cell and low current stability is
improved.
[0051] Instead of, or in addition to fuel cell remediation as
discussed above, the methods of the invention can be applied
prophylactically to make hydrophobic diffusion media that work more
reliably, especially when combined into fuel stacks containing 50,
100, 200, or more individual fuel cells.
[0052] Diffusion media prepared by coating with fluorocarbon
polymers tend also to gradually lose hydrophobicity over time
during stack operation. For example, over time, dark lines can be
observed forming on such carbon fiber diffusion media. The dark
lines represent areas less hydrophobic than the freshly coated
sample. Accordingly, water tends to accumulate at those areas of
the diffusion medium. In various embodiments, treating fluorocarbon
polymer coated diffusion media with hydrophobic silicone resin as
described herein mitigates this loss of hydrophobicity over time of
stack operation. Such resistance to aging can be observed, for
example in laboratory ex situ aging tests. In such aging tests,
coated diffusion media are exposed to 15% hydrogen peroxide
solution at 65.degree. C. for extended periods of time, for example
7 or 10 days. Wilhelmy measurements of advancing and receding
contact angles indicate response of the substrate to aging. In the
case of substrate dipped in silicone resin to apply the second
hydrophobic silicone coating, the contact angles tend to remain
constant or even go up slightly upon aging, reflecting good
retention of hydrophobicity upon accelerated aging.
[0053] The invention described above with respect to various
embodiments. Further non-limiting descriptions are given in the
working Examples that follow.
EXAMPLES
Example 1
Preparation of Fluorocarbon Polymer Coated Diffusion Medium
[0054] A fluoropolymer solution is prepared by diluting one part
DuPont T30 solution into 19 parts deionized water by volume. T30 is
a commercial product and is a dispersion of 60% by weight PTFE,
water, and surfactant. The diluted solution is stirred for about 5
minutes. Toray carbon paper (TGPH 060 or TGPH 090) is immersed into
the fluoropolymer solution for a time period of about 3 minutes up
to about 5 minutes. The paper is then removed form the solution and
allowed to drip dry for about 1 minute. After drip dry, the excess
solvent is removed from the paper by placing the paper in a
90.degree. C. oven until dry. The temperature is then ramped up to
290.degree. C. and then 380.degree. C. in the oven and held at the
respective temperatures for 30 minutes. The resulting diffusion
medium contains a hydrophobic fluorocarbon polymer adhered to the
substrate. The polymer coating on the substrate is sintered.
[0055] A number of carbon fiber substrates are fluorocarbon coated
as described in the preceding paragraphs. Although most of the
papers prepared in such a fashion exhibit suitable performance at
high relative humidity, an individual coated substrate having low
performance is identified and further tested below.
Example 2
[0056] The gas diffusion medium with low performance (poor water
management capability) identified in Example 1 is treated with a
silicone coating by dipping it into a dichloromethane solution
containing 2 gaiter Dow Corning 3140 adhesive. The substrate is
then air dried and installed into a fuel cell.
Example 3
[0057] A low performing diffusion medium identified from Example 1
and a diffusion medium further treated with silicone as described
in Example 2 are tested in fuel cells, with the results shown in
FIG. 2. The figures show current versus voltage curves. The fuel
cell is composed of a pair of serpentine graphite flow fields with
50 cm.sup.2 active area. The MEA used in the test is Gore 5510 (25
microns thick) MEA. Three different operating conditions are
tested: 1) cell temperature 70.degree. C., anode is pure H.sub.2,
cathode is air, the gas outlet is pressure 100 KPa (absolute), and
the inlet gas for the anode and cathode are under 40% relative
humidity that results in 84% outlet relative humidity during
operation; 2) cell temperature 80.degree. C., anode is pure
H.sub.2, cathode is air, the gas outlet is pressure 150 KPa
(absolute), and the inlet gas for the anode and cathode are under
66% relative humidity that results in 110% outlet relative humidity
during operation; and 3) cell temperature 60.degree. C., anode is
pure H.sub.2, cathode is air, the gas outlet is pressure 270 KPa
(absolute), and the inlet gas for the anode and cathode are under
100% relative humidity that results in 300% outlet relative
humidity during operation. At 84% outlet relative humidity, the
current versus voltage curves for the diffusion media of Example 1
and Example 2 are nearly identical.
[0058] FIGS. 2a and 2b show, respectively, the voltage curves for
comparative fuel cells run at 110% outlet relative humidity and
300% outlet relative humidity, respectively. The Figures show that
the cell performance of fuel cells containing silicone coated
diffusion media (Example 2) is superior to those having only the
fluorocarbon polymer coating (Example 1).
[0059] The description of the invention is merely exemplary in
nature and, thus, variations that do not depart from the gist of
the invention are intended to be within the scope of the invention.
Such variations are not to be regarded as a departure from the
spirit and scope of the invention.
* * * * *